We propose to extend small-angle scattering studies of the interactions between the Ca binding proteins calmodulin and troponin C and their regulatory targets. The systems we now wish to focus on have limited solubility resulting in the need for the most intense X-ray source available. A critical question to be answered is how well the previously used short peptide fragments have modeled interactions with intact target enzymes. The result will be a better understanding of the molecular basis for the Ca -dependent regulation in these important biochemical systems.